1. Field
The present disclosure relates to analog power supplies with a split mid-rail design. More particularly, it relates to split power supply bias with a kill switch.
2. Description of Related Art
Some electrical and electronic components and parts that use a supply voltage with a very wide operating range (for example, 2.3-5.5 V in cellphones) can require a low standby current. A voltage of 5.5 V can be too high to use as a typical cutoff switch in most low voltage processes. Thus, some active circuitry is required to drop this voltage to a more manageable voltage level before cutting off the supply voltage. However, when the supply is at the low end of its operating range (such as 2.3 V), this active circuitry can present excessive impedance and thus also an excessive voltage drop to the main chip because a voltage Vgs (a gate-to-source voltage of a transistor of the switch) to the cutoff switch (transistor) can be much less than 2.3 V due to the voltage limiting circuitry. Typical voltage-limiting circuitry may be similar to either a simple voltage regulator to drop 5.5 V down to within the process limits, or a mid-supply voltage generator which could be, for instance, a resistor divider with (or without) some form of buffering.
In the following, a high voltage of 5.5 V is used as an example to explain some embodiments of the present disclosure. As the person skilled in the art will understand, in other embodiments other voltages, higher or lower than 5.5 V, may be used.
As known to the person skilled in the art, power amplifier circuits (e.g. output stages) are classified as A, B, AB, C, D, etc. for analog designs. In class A amplifiers, 100% of the input signal is used and the active element remains conducting all of the time (i.e. full waveform). In class B amplifiers, 50% of the input signal is used: the active element carries current half of each cycle, and is turned off for the other half. Class AB is intermediate between class A and B, and the two active elements conduct more than half of the time, conducting during one half of the cycle and partially conducting over the other half.
In the case of limiting circuitry taking the form of a simple voltage regulator, the voltage regulator would require a cutoff switch following the regulator, and the mid-rail generator would require two stacked cutoff switches. A possible problem with the regulator is in creating the gate drive to the p-type metal-oxide-semiconductor (PMOS) low drop out (LDO) regulator: the high voltage has to be the high supply, whereas the low voltage side cannot be ground (GND) when the supply voltage (Vdd) is 5.5 V, but ideally wants to be near GND when Vdd is less than a corresponding process voltage limit. This presents significant problems to the design of an operational amplifier (opamp) which drives the LDO, as additional circuitry required to address such problems can use a significant amount of power.
Limiting circuitry taking the form of a mid-supply rail generator can encounter problems similar to that of the voltage regulator: For the mid-supply rail generator two cutoff switches may be required, each with a gate-source on-voltage (Vgs) which can be equal to half the Vdd voltage. For a Vdd of 2.3 V, there can be little overdrive (Vgs-Vt), which can force the use of very large PMOS devices to attain low on resistance (Ron).
The second part of the problem is the output voltage transition. Since Vdd can be greater than the process voltage limit, the voltage swing of the output of the circuit will also be greater than the process voltage. Therefore, the output swing may be too high compared to a safe process voltage. The cutoff switch itself, in that case, may have to be implemented as two distinct devices in series, such as, for example, devices (310, 350) in FIG. 3, and each device cannot exceed the process voltage limit. If both devices don't turn on or off equally, one device could get too much voltage. Some form of transition control is therefore required to ensure that all devices do not exceed their voltage limits. This requirement can apply for both the cutoff switches as well as any pull-down devices designed to null the output voltage.